The present invention relates generally to the field of switch mode power supply (SMPS). More particularly, embodiments of the present invention relate to circuits and methods for primary side control of a flyback switching power supply.
Switching power supplies have become increasingly important in today's technologies, from personal computers to portable electronic devices. Compared with traditional linear power supplies, switching mode power supplies have advantages such as small size, good stability, high efficiency, and low cost, etc. As a result, switching power supplies are commonly used in low-cost, low power applications. Among the many types of switching power supplies, flyback switching power supplies are widely used, in which a transformer is used to provide electrical isolation between input and output. The transformer in a SMPS usually has a primary winding, a secondary winding, and one or more of the auxiliary windings.
In secondary side control of flyback converter, signals from the output side are provided through an optical coupler to the primary side, where a controller uses the feedback signal to control the power switch. In contrast, in primary side control of flyback converter, the feedback signal is taken from an auxiliary winding on the primary side, thereby saving the cost of the optical coupler. When the primary power switch is turned off, an auxiliary transformer winding is used to reflect the secondary winding voltage. However, the voltage pulse induced on the auxiliary winding often includes an unstable ripple portion in the beginning of the pulse. Therefore, it is important to sample the feedback signal from the auxiliary winding at an appropriate time during the induced voltage pulse to obtain a feedback signal that is representative of the state of the power supply output.
Feedback voltage signal Vfb is usually sampled during the positive phase of the switching cycle, i.e., when Vfb is positive. This condition occurs when the power switch is turned off and currents are induced in the secondary winding and the auxiliary winding. A conventional sampling technique described in EP 1 146 630 A2 involves waiting for a fixed delay time after the positive feedback pulse is started before sampling the positive feedback pulse. Another conventional technique described in U.S. Pat. No. 7,102,889 involves measuring the duration of the positive feedback pulse in an earlier switching cycle, and sampling the feedback signal in the next switching cycle after a delay that is based on a fraction of the measured duration of the positive pulse in the previous switching cycle.
As described below, these conventional techniques tend to be inflexible and often increase the cost of the system. Therefore, improved techniques for generating the sampling signal in the primary side regulated flyback system are desirable.